US President George Bush might consider authorising the controversial interrogation method of waterboarding in future, the White House has said.

Belief that an attack was imminent and legal advice would be major factors in the decision, his spokesman said.

On Tuesday the CIA admitted for the first time that it had used the method, which simulates drowning and has been condemned as torture by rights groups.

Key Democrats are demanding a criminal inquiry into whether its use was legal.

Congress has been debating banning the CIA from using water-boarding, which involves strapping a suspect down and pouring water over their cloth-covered face to create the sensation of drowning.

White House spokesman Tony Fratto maintained that the practice - which the CIA said it had carried out on three high-profile al-Qaeda detainees - was legal.

There was the belief that additional catastrophic attacks against the homeland were inevitable, and we had limited knowledge about al-Qaeda and its workings - those two realities have changed

Michael HaydenCIA Director

"Torture is illegal. Every enhanced technique that has been used by the CIA... was brought to the department of justice and they made a determination that its use under specific circumstances and with safeguards was lawful," he said.

If the CIA wanted to water-board a suspect in future, the head of the CIA would discuss the particular circumstances with the attorney general, who would determine the legality of the issue, and then take it to the president, Mr Fratto said.

In December, the House of Representatives approved a bill that would ban the CIA from using harsh interrogation techniques such as water-boarding.

President Bush has threatened to veto the bill.

'Realities changed'

On Wednesday, Senator Dick Durbin, a senior Democrat, demanded that the US justice department open a criminal inquiry into the past use of water-boarding confirmed by CIA Director Michael Hayden on Tuesday.

Michael Hayden (R) spoke as Mike McConnell reported to Congress

Mr Hayden told Congress that water-boarding had only been used on three people, and not for the past five years.

One of those was Kuwaiti-born Khalid Sheikh Mohammed, accused of masterminding the 11 September attacks on the United States, he said.

The two others, both high-level al-Qaeda suspects, were Abu Zubaydah and Abd al-Rahim al-Nashiri, both from Saudi Arabia, Mr Hayden said.

"There was the belief that additional catastrophic attacks against the homeland were inevitable. And we had limited knowledge about al-Qaeda and its workings.